Optical Blast resisted

I am using the Observer's Optical Blast to interrupt MC'd people on Garrosh and I am finding it gets resisted quite often. This is not good, obviously. Anyone else have this problem and why is it happening?

Either hit cap, or he's seeing the "Immune" combat text pop up on his screen after using Optical Blast on an empowered MC, and he's confusing empowered MCs with normal MCs. (Normal MC's are stun / silencable, empowered ones aren't and are ONLY interruptable) The interrupt part of Optical Blast will still work and interrupt the cast, however you'll see "Immune" pop up on your screen in reference to the silence part of the spell. If the cast is actually being interrupted then don't fret about it all that much, you're doing your job. If not, then yeah, chances are you're not hit capped or something.

Inner Focus tooltip = "Reduces the mana cost of your next Flash Heal, Greater Heal or Prayer of Healing by 25% and increases its critical effect chance by 100%. Additionally, you become immune to Silence, Interrupt, and Dispel effects for 5 sec."

Priest is thus immune to Optical Blast

We're not sure if this is the case yet, the priest is going to let me know if the AI uses Inner Focus when he's MC'd

Inner Focus tooltip = "Reduces the mana cost of your next Flash Heal, Greater Heal or Prayer of Healing by 25% and increases its critical effect chance by 100%. Additionally, you become immune to Silence, Interrupt, and Dispel effects for 5 sec."

Priest is thus immune to Optical Blast

We're not sure if this is the case yet, the priest is going to let me know if the AI uses Inner Focus when he's MC'd

This is only true for the 3rd phase of the fight when all MCs are empowered. During the 2nd phase MCs you can stun them, to stop them from even casting Touch of Y'saarj in the first place. OP is claiming he's having the issue on even normal MCs, not just empowered ones.

@ OP, are you 100% certain it's NOT interrupting them at all, regardless of what the floating combat text says? It might just be a UI bug or something extremely weird and the UI is saying that it was resisted, but in reality it still interrupted the cast. If it really isn't interrupting, as someone else suggested, try switching between all of the Grimoires and seeing if it makes any difference at all. Having Sac'd interrupt vs. Observer interrupt vs. Felhunter interrupt.